Method of and apparatus for processing seismic data

ABSTRACT

A method determining a calibration filter to calibrate a first component of multi-component seismic data relative to a second component of the seismic data comprises determining the calibration filter from a portion of the seismic data that contains only events arising from critical refraction of seismic energy. The method is particularly suitable for long-off-set data, since the first arrival will be a critical refraction event and an automatic picking method may be used. The present invention also provides a wavenumber dependent calibration filter that is obtained from a calibration filter obtained from data in one offset range and another calibration filter obtained from data in another offset range.

The present invention relates to a method of processing multi-componentseismic data. It particularly relates to a method of processing seismicdata to determine a calibration filter that calibrates one component ofthe seismic data relative to another component of the seismic data. Theinvention further relates to an apparatus for processing seismic data.

FIG. 1 is a schematic view of a seismic surveying arrangement. In thisfigure the surveying arrangement is a marine surveying arrangement inwhich seismic energy is emitted by a seismic source 1 that is suspendedwithin a water column 2 from a towing vessel 3. When the seismic source1 is actuated seismic energy is emitted downwards and is detected by anarray of seismic receivers 4 disposed on the seafloor 5. (As used hereinthe term “seabed” denotes the earth's interior, and the term “seafloor”denotes the surface of the seabed.)

Many seismic surveys now use multi-component receivers that record twoor more components of the seismic energy incident on the receiver. Forexample a 3-component (3-C) seismic receiver contains three orthogonalgeophones and so can record the x-, y- and z-components of the particlemotion at the receiver (the particle motion may be the particledisplacement, particle velocity or particle acceleration or even, inprinciple, a higher derivative of the particle displacement). In amarine seismic survey a 4-component (4-C) seismic receiver canalternatively be used. A 4-C receiver contains a pressure sensor such asa hydrophone in addition to three orthogonal geophones and so can recordthe pressure of the water column (which is a scalar quantity) inaddition to the x-, y- and z-components of the particle motion.

Many different paths exist by which seismic energy may travel from thesource 1 to a receiver 4 in the seismic surveying arrangement of FIG. 1.A number of paths are indicated schematically in FIG. 1.

The path 6 shown in FIG. 1 is known as the “direct path”. Seismic energythat travels along the direct path 6 travels from the source 1 to areceiver 4 essentially in a straight line without undergoing reflectionat any interface.

Path 7 in FIG. 1 is an example of a “water layer multiple path”. Seismicenergy that follows a water layer multiple path propagates wholly withinthe water column 2, but undergoes one or more reflections at the surfaceof the water column and/or the seafloor 5 so that the seismic energypasses through the water column more than once. The water layer multiplepath 7 shown in FIG. 1 involves one reflection at the seafloor 5 and onereflection at the surface of the water column, but many other waterlayer multiple paths exist.

The path 8 in FIG. 1 is an example of a “critical refraction path”.Seismic energy that follows the path 8 propagates downwards to theseafloor 5, and penetrates into the earth's interior 10 (ie into theseabed). The seismic energy continues propagating downwardly, until itreaches a boundary 11 between two layers of the earth that havedifferent acoustic impedance. The seismic energy undergoes criticalrefraction, propagates along the boundary 11, before eventually beingrefracted upwards towards the receiver 4. Critical refraction may alsooccur at the water-seabed interface, and downwardly propagating seismicenergy that is refracted in this way will propagate along thewater-seabed interface and will then propagate upwardly into the watercolumn.

The path 9 shown in FIG. 1 is known as a “primary reflection path”.Seismic energy that follows the primary reflection path 9 propagatesdownwards through the water column, is refracted at the seafloor 5, andpropagates downwardly through the earth's interior. The seismic energyis refracted at the boundary 11, but is not critically refracted and socontinues to propagate downwardly into the earth. It eventuallyundergoes reflection at a geological structure 12 that acts as a partialreflector of seismic energy, and the reflected seismic energy is, afterfurther refraction as it passes upwardly through the boundary 11,incident on the receiver 4. The general intent of a seismic survey is tomake use of the seismic energy that follows the primary reflection pathin order to obtain information about the interior structure of theearth.

Seismic energy acquired at a receiver may contain upwardly and/ordownwardly propagating seismic energy depending on the location of thereceiver and on the event. For example seismic energy that travels alongthe critical refraction path 8 shown in FIG. 1 will, when it is incident(travelling upwardly) on the water-seabed interface, be partlytransmitted into the water column and partially reflected back into theseabed 10. Thus, a critical refraction event will consist purely ofupwardly propagating seismic energy above the seafloor 5, but willcontain both upwardly and downwardly propagating seismic energy belowthe seafloor 5. As another example, seismic energy that travels alongthe direct path 6 shown in FIG. 1 will, when incident on thewater-seabed interface 5, be partially transmitted into the seabed andpartially reflected back into the water column. Hence, the direct eventwill contain both upwardly and downwardly propagating seismic energyabove the seafloor, but will contain only downwardly propagating seismicenergy below the seafloor. It is therefore often of interest todecompose the seismic data acquired at the receiver 4 into an up-goingconstituent and a down-going constituent, above or below the seafloor 5.For example, in a 4-C seismic survey it may be of interest to decomposethe pressure and the vertical particle velocity recorded at the receiverinto their up-going and down-going constituents above the seafloor.Various filters that enable decomposition of seismic data into up-goingand down-going constituents have been proposed. One example can be foundin K. M. Schalkwijk et al, “Application of Two-Step Decomposition toMulti-Component Ocean-Bottom Data: Theory and Case Study”, J. Seism.Expl. Vol. 8 pp 261-278 (1999), and states that the down-going andup-going constituents of the pressure just above the seafloor may beexpressed as follows:

$\begin{matrix}{{{P^{-}\left( {f,k} \right)} = {{\frac{1}{2}{P\left( {f,k} \right)}} - {\frac{p}{2{q\left( {f,k} \right)}}{Z\left( {f,k} \right)}}}},{{P^{+}\left( {f,k} \right)} = {{\frac{1}{2}P\left( {f,k} \right)} + {\frac{p}{2{q\left( {f,k} \right)}}{Z\left( {f,k} \right)}}}},} & (1)\end{matrix}$where P is the pressure acquired at the receiver, P⁻ is the up-goingconstituent of the pressure above the seafloor, P⁺ is the down-goingconstituent of the pressure above the seafloor, f is the frequency, k isthe horizontal wavenumber, Z is the vertical particle velocity componentacquired at the receiver, p is the density of the water, and q is thevertical slowness in the water layer.

As can be seen, the expressions in equation (1) require two of thecomponents of seismic data recorded at the receiver to be combined.These filters are an example where it is necessary to combine twocomponents of the acquired seismic data. It may also be necessary tocombine two or more components of the acquired seismic data in order todecompose the acquired seismic data into p-wave and s-wave(pressure-wave and shear-wave) components, or to remove water levelmultiple events from the seismic data.

One problem in combining different components of the seismic dataacquired at a receiver is that the different components of the seismicdata may not be correctly calibrated against one another. This isparticularly the case where the two components that are being combinedare, as in equation (1), the pressure and the vertical particlevelocity. There are usually differences in coupling or impulse responsebetween the hydrophone used to acquire the pressure and the geophoneused to acquire the vertical particle velocity. It is necessary tocalibrate the data for these differences before the pressure andvertical particle velocity can be combined. This may be done bydeveloping a calibration filter that compensates for the differences incoupling and impulse response between the hydrophone and the verticalgeophone.

Schalkwijk et al, and others, have suggested that the calibrationproblem can be addressed by assuming that one component of the seismicdata has been correctly recorded, and calibrating the other component ofthe seismic data against the component that is assumed to be correctlyrecorded. In general, it is assumed that the hydrophone is well coupled,so that the pressure recording is taken to be correct. The verticalcomponent of the particle velocity is then calibrated against thepressure to compensate for coupling and impulse response differencesbetween the hydrophone and the vertical geophone. Schalkwijk et altherefore proposed that equation (1) above should be modified byapplying a calibration filter to the vertical particle velocity. Theyproposed that the equation given above for the down-going constituent ofthe pressure above the seafloor should be modified to read as follows:

$\begin{matrix}{{P^{+}\left( {f,k} \right)} = {{\frac{1}{2}P\left( {f,k} \right)} + {{a(f)}\frac{p}{2{q\left( {f,k} \right)}}{{Z\left( {f,k} \right)}.}}}} & (2)\end{matrix}$In equation (2) a(f) represents a frequency-dependent calibrationfilter. The remaining terms in equation (2) have the same meaning as inequation (1).

The method proposed by Schalkwijk et al for determining the calibrationfilter a(f) is to minimise the energy of the down-going pressureconstituent above the seafloor for a portion of the seismic data thatcontains only primary reflections. Seismic energy travelling along aprimary reflection path is propagating upwardly just above the seafloorat the receiver position, so that the down-going constituent of thepressure just above the seafloor should be zero for data that containsonly primary reflections. Schalkwijk proposed that the calibrationfilter that minimises the energy of the down-going pressure in a windowcontaining only primary reflection events can be found using a leastsquares method. Once the calibration filter a(f) has been determined inthis way, it is applied to the entire data set for the vertical particlevelocity.

The existence of various paths of seismic energy from the source to thereceiver means that the data acquired at the receiver in a real seismicsurvey will contain events corresponding to more than one possible path.These events will occur at different times after the actuation of theseismic source 1, as different paths of seismic energy have differentassociated travel times. FIG. 2 is a schematic illustration of seismicdata that might be acquired at the receiver 4, and it shows theamplitude of seismic energy recorded at the receiver 4 as a function ofthe time since the actuation of the source 1. FIG. 2 illustrates adirect event 13, corresponding to the direct path 6, a criticalrefraction event 14 corresponding to the critical refraction path 8, aprimary event 15 corresponding to the primary reflection path 9, and awater layer multiple event 16 corresponding to the water layer multiplepath 7. (In practice, data acquired at a receiver will contain aplurality of primary reflection events from different geologicalstructures, a plurality of critical refraction events, and a pluralityof water level multiple events arising from different water levelmultiple paths. Only one event of each type is shown in FIG. 2 forsimplicity of explanation.) In order to apply the method of Schalkwijket al to determine the calibration filter, data in a time window thatcontains only the primary event 15, such as the time window A shown inFIG. 2, must be selected.

The present invention provides a method of processing multi-componentseismic data obtained from seismic signals propagating in a medium, themethod comprising the steps of: selecting a first portion of the seismicdata containing only events arising from critical refraction of seismicenergy; and determining a first calibration filter from the firstportion of the seismic data, the first calibration filter being tocalibrate a first component of the seismic data relative to a secondcomponent of the seismic data.

The method proposed by Schalkwijk et al has the disadvantage that thetime window containing only primary reflection events has to be pickedmanually. The primary reflection events are not the first eventsacquired at the receiver following actuation of the source, and socannot be picked automatically. A further disadvantage is that in somecases, for example if the seismic source has a long signature, it may behard to distinguish between the direct arrival and the primaryreflection events, so that it may be difficult to isolate the correctevents. The direct event contains downwardly propagating seismic energyso that use of a time window that inadvertently included the directevent would not give correct results for the calibration filter, sincethe method for determining the calibration filter assumes that theselected data contains only up-going energy. A further problem with themethod of Schalkwijk et al is that in shallow water the water layermultiple events may arrive at substantially the same time as the primaryreflection events, and this again makes it difficult to pick a timewindow that includes only the primary reflection events.

The present invention makes use of the fact that the critical refractionevents consist only of up-going seismic energy just above the seafloor.Thus, selecting a time window that contains only one or more criticalrefraction events makes it possible to determine the calibration filtera(f) by the technique of minimising the energy of the down-goingpressure just above the seafloor in that time window.

The method of the invention is particularly advantageous when applied tolong offset data. As is shown in FIG. 3, as the offset (that is, thehorizontal distance between the source and the receiver) increases, thearrival time of the first critical refraction event increases moreslowly than does the arrival time of the direct event. For offsetsgreater than O₁ the first arrival at the receiver is not the directevent, but is the critical refraction event. That is, at long offsetsthe critical refraction event 14 in FIG. 2 (two critical refractionevents are shown in FIG. 3) will arrive before the direct event 13 andwill be the first arrival at the receiver. When the invention is appliedto data having a source-receiver offset sufficiently large for the firstevent acquired at the receiver to be a critical refraction event, it ispossible to use a time window that covers only the first event acquiredat the receiver—and this makes it possible to use an automatic pickingmethod to determine the time window. If several critical refractionevents arrive at the receiver before the direct event, as for faroffsets in FIG. 3 where two critical refraction events arrive before thedirect event arrives, then all these critical refraction events may beincluded in the time window.

FIG. 4 is a schematic illustration that corresponds to FIG. 2, butillustrates the arrival times of the events at an offset that issufficiently large such that the first arrival is a critical refractionevent. In this case, the invention may be applied by selecting the timewindow B which includes the critical refraction event only, andminimising the energy of the down-going pressure above the seafloor inthis time window.

A further advantage of the invention is that the method may be appliedto seismic data acquired in shallow waters. Although water layermultiple events in seismic data acquired in shallow water may coincidewith primary reflection events, they do not coincide with criticalrefraction events. Choosing a time window that includes only thecritical refraction event therefore ensures that the time window cannotcontain water layer multiple events. The invention also overcomes theproblems that arise when a seismic source having a long source signatureis used.

A preferred embodiment of the invention comprises the further steps ofselecting a second portion of the seismic data containing only eventsarising from primary reflection of seismic energy and determining asecond calibration filter from the second portion of the seismic data,the second calibration filter being to calibrate the first component ofthe seismic data relative to the second component of the seismic data.It may comprise the further step of determining a wavenumber-dependentcalibration filter from the first calibration filter and the secondcalibration filter.

A further problem related to the method proposed by Schalkwijk et al isthat the correct calibration filter a(f) may well be dependent on thewavenumber as well as on the frequency. The calibration filter proposedby Schalkwijk, however, is dependent only on frequency and, furthermore,is derived purely from seismic data at low wavenumbers. In an embodimentof the present invention, the filter obtained from the criticalrefraction events is combined with a filter obtained from primaryreflection events, and a wavenumber-dependent filter is obtained fromthe two individual filters. The wavenumber-dependent filter may beobtained by, for example, interpolation between the filter derived fromthe critical refraction events and the filter derived from the primaryreflection events.

A second aspect of the present invention provides a method of processingmulti-component seismic data obtained from seismic signals propagatingin a medium, the method comprising the step of selecting a first portionof the seismic data corresponding to a first wavenumber range;determining a first calibration filter from the first portion of theseismic data; selecting a second portion of the seismic datacorresponding to a second wavenumber range different from the firstwavenumber range; determining a second calibration filter from thesecond portion of the seismic data; and determining awavenumber-dependent calibration filter from the first calibrationfilter and the second calibration filter, the wavenumber-dependentcalibration filter being to calibrate a first component of the seismicdata relative to a second component of the seismic data.

A third aspect of the invention provides a method of processingmulti-component seismic data obtained from seismic signals propagatingin a medium, the method comprising the steps of: selecting a firstportion of the seismic data in which the first arrival contains onlyupwardly propagating seismic energy above the seafloor; and determininga first calibration filter from the first portion of the seismic data,the first calibration filter being to calibrate a first component of theseismic data relative to a second component of the seismic data.

The invention may be applied to any event that is the first arrival andthat contains only upgoing energy above the seafloor. For example, atfar offsets the first arrival may be an event which is not a criticalrefraction event but which nevertheless contains only upgoing energyabove the seafloor—such as, for example, a wave that was trapped in athin subsurface layer of the seabed—and the invention may be applied tosuch events.

The invention may further comprise the step of calibrating the firstcomponent of the seismic data using the first calibration filter orusing the wavenumber-dependent calibration filter.

A fourth aspect of the present invention provides a method of seismicsurveying comprising the steps of: actuating a source of seismic energy;acquiring seismic data at a receiver spatially separated from thesource; and processing the seismic data by a method as defined above.

A fifth aspect of the present invention provides an apparatus forprocessing multi-component seismic data to determine a calibrationfilter to calibrate a first component of the seismic data relative to asecond component of the seismic data, the apparatus comprising: meansfor selecting a first portion of the seismic data containing only eventsarising from critical refraction of seismic energy; and means fordetermining a first calibration filter from the first portion of theseismic data. The apparatus may comprise a programmable data processor.

A sixth aspect of the invention provides an apparatus for processingmulti-component seismic data to determine a calibration filter tocalibrate a first component of the seismic data relative to a secondcomponent of the seismic data, the apparatus comprising: means forselecting a first portion of the seismic data in which the first arrivalcontains only upwardly propagating seismic energy above the seafloor;and means for determining a first calibration filter from the firstportion of the seismic data.

A seventh aspect of the invention provides an apparatus for processingmulti-component seismic data to determine a calibration filter tocalibrate a first component of the seismic data relative to a secondcomponent of the seismic data, the apparatus comprising: means forselecting a first portion of the seismic data corresponding to a firstwavenumber range; means for determining a first calibration filter fromthe first portion of the seismic data; means for selecting a secondportion of the seismic data corresponding to a second wavenumber rangedifferent from the first wavenumber range; means for determining asecond calibration filter from the second portion of the seismic data;and means for determining a wavenumber-dependent calibration filter fromthe first calibration filter and the second calibration filter.

The apparatus may comprise a programmable data processor.

An eighth aspect of the present invention provides a storage mediumcontaining a program for an apparatus as defined above.

The invention also provides a method of determining a first calibrationfilter for calibrating a first component of multi-component seismic datarelative to a second component of the seismic data, the methodcomprising the steps of: selecting a first portion of the seismic datacontaining only events arising from critical refraction of seismicenergy; and determining the first calibration filter from the firstportion of the seismic data.

The invention also provides a method of determining awavenumber-dependent calibration filter for calibrating a firstcomponent of multi-component seismic data relative to a second componentof the seismic data, the method comprising the steps of: selecting afirst portion of the seismic data corresponding to a first wavenumberrange; determining a first calibration filter from the first portion ofthe seismic data; selecting a second portion of the seismic datacorresponding to a second wavenumber range different from the firstwavenumber range; determining a second calibration filter from thesecond portion of the seismic data; and determining awavenumber-dependent calibration filter from the first calibrationfilter and the second calibration filter.

The invention also provides a method of determining a first calibrationfilter for calibrating a first component of multi-component seismic datarelative to a second component of the seismic data, the methodcomprising the steps of: selecting a first portion of the seismic datain which the first arrival contains only upwardly propagating seismicenergy above the seafloor; and determining a first calibration filterfrom the first portion of the seismic data.

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying figuresin which:

FIG. 1 is a schematic illustration of a seismic survey;

FIG. 2 is a schematic illustration of the seismic energy acquired at areceiver in the seismic survey of FIG. 1;

FIG. 3 is a schematic illustration of the variation of arrival time ofseismic energy as a function of offset between the source and thereceiver;

FIG. 4 is a schematic illustration of the seismic energy acquired in theseismic survey of FIG. 1 at long offsets, illustrating a method of thepresent invention;

FIG. 5 is a schematic illustration of pressure recorded at a receiver inthe seismic surveying arrangement of FIG. 1;

FIGS. 6 and 7 illustrate the up-going and down-going constituents ofpressure above the seafloor obtained from the pressure data shown inFIG. 5 according to a prior art approach;

FIGS. 8 and 9 illustrate the up-going and down-going pressureconstituents above the seafloor obtained from the pressure data of FIG.5 according to a method of the present invention;

FIG. 10 is a schematic block flow diagram of a method of the presentinvention; and

FIG. 11 is a block schematic diagram of an apparatus according to thepresent invention.

FIG. 5 illustrates typical pressure data recorded at a 4-C receiver in aseismic survey such as the survey shown in FIG. 1. The x-axis in FIG. 5indicates the offset between the source and the receiver, and the y-axisindicates the time after actuation of the seismic source. The data arecommon receiver data and were acquired using a single receiver and alinear array of sources deployed with a spacing of 50 m between eachpair of adjacent sources. Each trace represents the pressure acquired atthe receiver when one source is actuated, with the amplitude of theacquired pressure being in the x-direction.

It should be noted that different receivers in an array may well havedifferent coupling, different instruments responses etc, even if all thereceivers are nominally identical to one another. The calibration filterrequired for data acquired at one receiver in a receiver array istherefore likely to be different from the calibration filter requiredfor data acquired at another receiver in the array. The invention istherefore preferably applied to common receiver gathers and a separatecalibration filter is determined for each common receiver gather.

The pressure data shown in FIG. 5 contains a large number of seismicevents. The event labelled 13 is the direct wave, and it will be seenthat this is the first arrival for offsets having a magnitude of up toapproximately 1,000 m. The event labelled 14 is a critical refractionevent, and it will be seen that this is the first arrival for offsetshaving a magnitude significantly greater than 1,000 m.

FIGS. 6 and 7 illustrate the up-going constituent above the seafloor(FIG. 6) and the down-going constituent above the seafloor (FIG. 7) ofthe pressure shown in FIG. 5 obtained using the filters given inequation (1) above. That is, the up-going and down-going constituentshown in FIGS. 6 and 7 were obtained on the assumption that the pressuredata and the vertical particle velocity data (not illustrated) werecorrectly calibrated to one another. Inspection of FIGS. 6 and 7 showsthat this assumption is incorrect. In particular, the criticalrefraction event 14 contains only up-going energy above the seafloor andso should appear only in the up-going pressure constituent and shouldnot appear in the down-going pressure constituent. It will, however, beseen that the up-going critical refraction event has leaked through intothe down-going pressure constituent shown in FIG. 7, and this indicatesthat the calibration is unsatisfactory.

According to the present invention, a calibration filter is determinedfrom the critical refraction event 14. As noted above, for tracesacquired at a source-receiver offset having a magnitude well above 1000m, the critical refraction event is the first event acquired at thereceiver, and is well-separated from the subsequent event. It istherefore possible for such traces to define a time-offset window thatincludes only the first critical refraction event, and so includes onlyup-going energy.

One suitable time-offset window of data is illustrated in FIG. 5, as theregion C. It will be seen that this region includes traces acquired atan offset of between −3000 m to approximately −2100 m. For each trace inthis offset range the region C defines a time window that includes onlythe first refraction event (which is the first arrival in each of theselected traces). It will be noted that the center point of the timewindow for a particular trace is not constant between traces butincreases with increasing magnitude of offset.

The calibration filter for the vertical velocity component is thencalculated on the assumption that the energy in the selected portion Cof the data should contain only up-going energy. The calibration filtermay be determined in any suitable way. In particular, the calibrationfilter a(f) may be determined by finding the calibration filter thatminimises the energy of the down-going pressure constituent using aleast squares process, as in the method of Schalkwijk et al. Once theappropriate calibration filter a(f) has been determined, revised filtersfor determining the up-going and down-going constituents of the pressureabove the seafloor can be determined using equation (2), or in generalthe filter a(f) may be applied to the entire gather of the verticalcomponent data, and the calibrated vertical component data can then beused as an input to any process requiring a combination of the verticalcomponent with any other seismic components.

Another suitable portion of data exists in the corresponding region forpositive offsets in the range 2100 to 3000 m. One possibleimplementation of the method would be to use both these regions, bydefining a second region, analogous to the region C in FIG. 5, foroffsets in the range +2100 m to +3000 m and determining a secondcalibration filter. The two filters determined from the two windows maythen be averaged. This will however not be possible for all data setssince a receiver gather does not necessarily have the same amount ofpositive and negative offsets, and hence a region with a clearlyseparated critical refraction event may be present only for eitherpositive or negative offsets.

FIGS. 8 and 9 illustrate the results of decomposing the pressure data ofFIG. 5 into its up-going and down-going constituents above the seafloorusing filters of the type given in equation (2) above, and with thecalibration filter a(f) determined from the seismic data in the region Cof FIG. 5. It will be noted that the critical refraction event 14appears predominantly in the up-going pressure constituent of FIG. 8,and is almost completely absent from the down-going pressure constituentof FIG. 9. This illustrates that the decomposition of FIGS. 8 and 9 issignificantly more accurate than the decomposition of FIGS. 6 and 7,since the critical refraction event is expected to occur only in theup-going pressure constituent.

It will also be noted that the primary reflection event is stronger inthe up-going pressure constituent of FIG. 8 than in the up-goingpressure constituent of FIG. 6. This suggests that the calibrationfilter found from the critical refraction event at long offsets is alsoapplicable at low offsets.

FIG. 10 is a block flow diagram illustrating one embodiment of themethod of the present invention.

Initially, at step 17, seismic data is acquired. This may be, forexample, acquired in a survey of the type shown in FIG. 1.

The invention may alternatively be applied to pre-existing seismic data.Step 17 may therefore be replaced by the alternative step 18 ofretrieving pre-existing seismic data from storage.

At step 19 a suitable offset range is selected. In the example describedabove with reference to FIG. 5, step 19 consists of selecting the offsetrange from −3,000 m to −2100 m.

At step 20, the first arrival of seismic energy for each trace in theselected offset range is determined (this may be thought of a selectinga time window for each trace, and so defining an offset-time window).Assuming that the offset range has been selected correctly in step 19,the first arrival in each trace in the selected offset range will be acritical refraction event such as the event 14. Since the event is thefirst event in each trace, step 20 may be carried out using an automaticpicking method, although it may alternatively be performed manually.

At step 21, a calibration filter is determined that is the best fit tothe data in the selected offset range and time window. This is done bycalculating the down-going pressure constituent above the seafloor fromthe pressure and vertical particle velocity recorded at the receiverusing equation (2), and finding the calibration filter that minimisesthe energy in the down-going constituent of the pressure.

At step 22 the filter a(f) is applied to all the desired traces of thevertical component of the seismic data acquired at step 17 or retrievedfrom storage at step 18.

At step 23 the calibrated vertical component data is then used as inputinto any process requiring a combination of several seismic components.For example, filters for determining the up-going and down-goingconstituents of the pressure above the seafloor may be determined, usingequation (2) and the calibration filter determined at step 21.

If desired, steps 22 and 23 may be omitted. In this case the calibrationfilter determined at step 21 may be output for display or stored forsubsequent use.

In an alternative embodiment of the invention, a wavenumber-dependentfilter is determined by combining the approach of Schalkwijk et al withthe present invention. In this embodiment, a calibration filter isdetermined from critical refraction events occurring at long offsets, asdescribed above with reference to steps 17 to 21 of FIG. 10. A secondcalibration filter is then determined from traces in which it ispossible to define a time-offset window that contains only primaryreflection events. A calibration filter is determined for these tracesin the manner described by Schalkwijk et al. A suitable region of datafor obtaining this filter is indicated on FIG. 5 as region D.

The calibration filter determined from critical refraction events atlong offset and the calibration filter determined from primaryreflection events at low offset are then combined to produce awavenumber-dependent calibration filter. The filters may be combinedusing an interpolation technique to determine the filter to be appliedat a given offset.

In this embodiment, step 22 of FIG. 10 is replaced by the step ofcalibrating the vertical component using the wavenumber-dependentcalibration filter. Alternatively steps 22 and 23 may be omitted, andthe wavenumber-dependent calibration filter can be output or stored forfuture use.

An alternative way to obtain a wavenumber-dependent calibration filteris to compute a calibration filter for each separate trace in the offsetrange selected at step 19. In this alternative embodiment steps 20 and21 are performed on each trace (or on a plurality of selected traces) inthe offset range selected at step 19 so that calibration filters aredetermined for several different wavenumbers. Alternatively, the tracesin the offset range selected at step 19 can be grouped, and acalibration filter can be determined for each group of traces, forexample using a least squares method. Again, this results in calibrationfilters for several different wavenumbers.

Once calibration filters have been obtained for several differentwavenumbers, it is possible to interpolate between and/or extrapolatefrom these calibration filters to obtain a wavenumber-dependentcalibration filter. This method would, however, only work well for atime-offset window containing only primary reflections (i.e., the windowD in FIG. 5), since the vertical slowness is constant for the refractedevent. The wavenumber-dependent calibration filter may be usedimmediately, or may be output or stored for future use.

A further alternative method is to define time-offset windows aroundseveral refraction events of different vertical slownesses, anddetermine a plurality of calibration filters (one calibration filter canbe obtained from data in each window). A wavenumber-dependentcalibration filter can be obtained by interpolation between and/orextrapolation from these calibration filters. If desired, one or morecalibration filters determined from a time-offset window containing onlyprimary reflections can also be used in the interpolation and/orextrapolation. The wavenumber-dependent calibration filter again may beused immediately, or may be output or stored for future use.

It will be noted in FIG. 5 that the primary reflection event is obscuredby other events at long offsets. It will therefore be extremelydifficult to compute a reliable calibration filter at long offsets usingthe method of Schalkwijk et al, owing to the difficulty of determining atime window that contains only primary reflection events. Furthermore,even if a time window that contained only primary reflection eventscould be determined for the long offset traces in FIG. 5, this couldonly be done by a manual picking method and could not be automated.

The invention has been described above with reference to a calibrationfilter that calibrates the vertical particle motion with regard to thepressure, on the assumption that the pressure has been accuratelyrecorded. The invention is not limited to this, however, and inprinciple could be used to determine a calibration filter thatcalibrates the pressure with regard to the vertical particle motion, onthe assumption that the vertical particle motion has been accuratelyrecorded.

FIG. 11 is a schematic block diagram of an apparatus 34 according to thepresent invention. The apparatus is able to carry out a method accordingto the present invention.

The apparatus 34 comprises a programmable data processor 27 with aprogram memory 28, for instance in the form of a read only memory ROM,storing a program for controlling the data processor 27 to processseismic data by a method of the invention. The apparatus furthercomprises non-volatile read/write memory 29 for storing, for example,any data which must be retained in the absence of power supply. A“working” or “scratchpad” memory for the data processor is provided by arandom access memory (RAM) 30. An input device 31 is provided, forinstance for receiving user commands and data. An output device 32 isprovided, for instance for displaying information relating to theprogress and result of the method. The output device may be, forexample, a printer, a visual display unit or an output memory.

Seismic data for processing may be supplied via the input device 31 ormay optionally be provided by a machine-readable store 33.

The program for operating the apparatus and for performing a method asdescribed hereinbefore is stored in the program memory 28, which may beembodied as a semi-conductor memory, for instance of the well-known ROMtype. However, the program may be stored in any other suitable storagemedium, such as magnetic data carrier 28 a (such as a “floppy disc”) orCD-ROM 28 b.

1. A method of processing multi-component seismic data obtained fromseismic signals propagating in a medium, the method comprising the stepsof: selecting a first portion of the seismic dab containing only eventsarising from critical refraction of seismic energy; and determining afirst calibration filter from the first portion of the seismic data, thefirst calibration filter being to calibrate a first component of theseismic data relative to a second component of the seismic data.
 2. Amethod as claimed in claim 1 wherein the first portion of the seismicdata is data acquired with a long source-receiver offset.
 3. A method asclaimed in claim 1 wherein the first component is the vertical componentof particle motion and the second component is pressure.
 4. A method asclaimed in claim 1 wherein the first component is pressure and thesecond component is the vertical component of particle motion.
 5. Amethod as claimed in claim 1 wherein the step of determining the firstcalibration filter comprises minimising the energy immediately above theseafloor of the downgoing constituent of the second component for theselected portion of the seismic data.
 6. A method as claimed in claim 1and comprising the further steps of selecting a second portion of theseismic data containing only events arising from primary reflection ofseismic energy and determining a second calibration filter from thesecond portion of the seismic data, the second calibration filter beingto calibrates the first component of the seismic data relative to thesecond component of the seismic data.
 7. A method as claimed in claim 6and comprising the further step of determining a wavenumber-dependentcalibration filter from the first calibration filter and the secondcalibration filter.
 8. A method as claimed in claim 7 and comprising thefurther step of calibrating the first component of the seismic datausing the wavenumber-dependent calibration filter.
 9. A method asclaimed in claim 1 and comprising the further step of calibrating thefirst component of the seismic data using the first calibration filter.10. A method of seismic surveying comprising the steps of: actuating asource of seismic energy; acquiring seismic data at a receiver spatiallyseparated from the source; and processing the seismic data by a methodas defined in claim
 1. 11. A method of processing multi-componentseismic data obtained from seismic signals propagating in a medium, themethod comprising the steps of: selecting a first portion of the seismicdata corresponding to a first wavenumber range; determining a firstcalibration filter from the first portion of the seismic data; selectinga second portion of the seismic data corresponding to a secondwavenumber range different from the first wavenumber range, wherein thefirst wavenumber range corresponds to seismic data containingsubstantially only critical refraction events and the second wavenumberrange corresponds to seismic data containing substantially only primaryreflection events; determining a second calibration filter from thesecond portion of the seismic data; and determining awavenumber-dependent calibration filter from the first calibrationfilter and the second calibration filter, wherein thewavenumber-dependent calibration filter is configured to calibrate afirst component of the seismic data relative to a second component ofthe seismic data.
 12. An apparatus for processing multi-componentseismic data to determine a calibration filter for calibrating a firstcomponent of the seismic data relative to a second component of theseismic data, the apparatus comprising: means for selecting a firstportion of the seismic data containing only events arising Dom criticalrefraction of seismic energy; and means for determining a firstcalibration filter from the first portion of the seismic data.
 13. Anapparatus as claimed in claim 12 and further comprising means forcalibrating the first component of the seismic data using the firstcalibration filter.
 14. An apparatus as claimed in any of claims 12 andcomprising a programmable data processor.
 15. A storage mediumcontaining a program for an apparatus as defined in claim 14.